Balanced Audio Technology VK80i Integrated Amplifier

This is a big deal. Balanced Audio Technology’s first all-tube integrated is a stunning piece of industrial art that delivers over 55Wpc of pure triode power. The design combines two of my favorite tubes, the 6SN7 dual triode (actually in this case the 6H8C Russian equivalent), and the unique 6C33C-B power triode, originally designed for avionics applications and famously used as a regulator tube in the radio communications system of the MIG-25 jet fighter. Its thick glass envelope is responsible for much of its weight. There are two internal sections with individual heaters, but their grids and plates are strapped in parallel. The only U.S. near-equivalent is the 6336, which isn’t nearly as robust.

One reason beam power and pentode designs became so popular in the 1950s and 60s was due to their much greater power output relative to triode-based designs. The ability to deliver 60 watts from a push-pull pair of power tubes was back then a major marketing advantage. The 6C33C-B changes the power calculus when it comes to triodes. Designer Victor Khomenko, whose initials are embedded in the model’s name, tells me that the VK80i is spec’d conservatively and is typically capable of 75–80Wpc. That should be plenty of power for the average domestic loudspeaker.

Production of the 6C33C-B at the Ulyanovsk factory stopped long ago, circa 1992. VK revealed that peak production was around 70,000 tubes a month, and that today there are still tons of them in warehouses—so, supply is not a problem. BAT maintains a sizable stock on hand, and as they test them, they reject close to 30% of tubes for various reasons, which includes failure to closely meet tube parameters. For this reason, BAT recommends that output tubes only be purchased from the factory. It’s worth mentioning that many years ago VK was at the Ulyanovsk factory to discuss the possibility of making bigger version of the 6C33C-B, but they were not interested in manufacturing only a few thousand pieces. VK also mentioned the 7241, which is a 100W regulator triode. He had samples and actually built a prototype using it, which sounded superb, but the tube was too delicate and would often die on heavy transients. In contrast, the 6C33-B is extremely robust and takes transients in stride.  

 With the VK-80i, the focus is on simplicity of operation and reliability. For starters, the VK-80i incorporates automatic-bias circuitry that compensates for line voltage fluctuations and tube aging. Each output tube has its own dedicated bias circuit. When first powered on, the amp undergoes a soft start-up, followed by bias checks, after which the amp is ready to boogie. It sounds fine after a couple of minutes but really comes into its own after about 30 minutes. Instead of using rail fuses to protect the output stage, the amp features two levels of protection.  One is thermal protection, which guards against over-stressing the unit. As VK explains, the “problem is that these amps don’t clip in an obvious, unpleasant way, like some more traditional amps do; so, people sometimes run them in their red zone without realizing it. The thermal protection guards against this.” In addition, the currents through each of the four output tubes are continuously monitored. Should the current exceed a preset value, that channel is disabled. This circuit uses a large power MOSFET as a switch element, and it offers much better performance than a traditional fuse. With one difference, this is the same protection circuit that BAT developed for use in the top-of-the-line REX power amp and is said to be totally transparent sonically. The only difference is that in the REX each tube has its own disable circuit, while in the VK-80i the same power MOSFET disables both tubes in the affected channel. After a protection circuit is tripped, all you need to do is turn the VK-80i off and on again to restore normal operation. There are no output tube fuses to replace or bias pots to adjust—ever!

The front-end circuit consists of two long-tailed differential gain stages. One balanced XLR input and three unbalanced RCA inputs are provided on the back panel. For a single-ended input, the first gain stage acts as a phase splitter, and from that point on, the signal is balanced. The output resistance is somewhat dependent on the choice of output taps, but is around 1 ohm. VK prefers that type of specification over the term “damping factor,” which is often misunderstood since the actual damping factor must include the woofer’s voice-coil resistance. That aside, the conventional damping factor is about 8 with respect to an 8-ohm load. Only about 3dB of negative global feedback (NFB) is used, which helps to control the amp’s source impedance, thus making it a bit less speaker dependent. And even more importantly, the low amount of NFB makes it extremely stable into highly reactive loads such as electrostatic speakers.

I made the decision to treat the VK80i much like a power amp and positioned it between the speakers so I could continue using short speaker cables and long interconnects. But there were unintended consequences. It turned out that the Denafrips Terminator DAC wasn’t happy driving long interconnects. I had to insert a tube buffer ahead of the VK80i to restore dynamics to their full glory. After the setup was complete, I found myself in my listening seat with the unit’s attractive metal-clad remote control in hand. The remote is able to duplicate all of the front-panel functions, including on/off, input selection, mute, and volume control. The only design feature missing is a balance control, something ignored by many other preamps these days.

It should also be mentioned that it’s vital to experiment with the impedance tap selections. There are three possible choices, as denoted on the back panel: High (6–8 ohms), Med (4–6 ohms), and Low (3–4 ohms). As the manual points out, some experimentation may be required to achieve the best tonal balance, and since there is no risk of damage, investigation is encouraged. As VK points out, the reason for this is that there is no single simple relationship that one could follow in tap selection, and it’s not necessarily just about maximum power transfer. He says that he himself has used different taps depending upon his particular situation at the time. BAT’s rough guidance is that the Low taps tend to provide better bass grip, while the higher ones produce more of an open sound.  

Folks, it felt like I needed to buckle up and hang on tight, as this “BAT mobile” sounded far more powerful than a comparable KT88 amp. It went through its gears without changing its sonic character. In fact, it was hard to tell when it redlined. There was no perceptible textural grain or brightness even when it was driven hard. Of course, from a measurement standpoint THD increased with power output, but the distortion spectrum was sufficiently benign, meaning even-order products, that my auditory system didn’t complain. 

The BAT was adept at fleshing out the power range of an orchestra, which would be the lower midrange. The bass range was quite good, but lacked the visceral impact of a high-power solid-state amp such as the Krell KST-100, which I keep around as a reference for its stupendous bass impact and definition. The difference was obvious on tympani strikes, but in deference to the BAT, it should be noted that it excelled when compared to other tube amps. The core of the midrange was expressive and dynamically nuanced, felicitous of female voice and violin overtones. Transients were well defined over their entire time scale from the point of attack to full decay. As a consequence, resolution of low-level detail was excellent, without being etched or analytical in nature.

It all came together for me on a wonderful recording of the Brahms Violin Concerto in D Major with Itzhak Perlman and Carlo Maria Giulini conducting the Chicago Symphony Orchestra. There is no shortage of great performances of this work, as I discovered through Qobuz streaming, but this 1976 recording at Chicago’s Medinah Temple stands out in my mind as locking in the right tempo and capturing the epic sweep of this work most perfectly. The BAT proved adept at portraying a transparent and believable impression of space while also capturing tonal colors with admirable fidelity.
 The BAT was free from the usual tube-amplification colorations. There was none of the excessive second-order harmonic richness exhibited by so many single-ended triode amps (SET). Its overall presentation was neutral, sounding like the real thing with believable tonal color saturation and without much editorializing. Neither was it overly soft sounding like so many bandwidth-limited SETs. But its tube origins were unmistakable when it came to soundstage dimensionality. And it could sound deliciously sweet when the music demanded it. 

I was recently reunited with the Audiostatic ES-240 electrostatics, speakers that I had lived with during the early 1980s. Richard J. Sabella reviewed the ES-240 in the December 1981 issue of TAS and struggled to make them image properly in his small room. That was not a problem in my much bigger listening space. The catch is that they’re extremely inefficient, and should ideally be driven by a high-power amp in the 200Wpc range, but I thought I would give the BAT a try. Output levels were limited, but I was surprised by how sweet the BAT sounded in the context of a difficult load.

Next in line for the BAT was the Innersound Isis 3.5 electrostatic hybrid, one of the most cohesive hybrids I’ve ever auditioned, and a challenging capacitive load in the treble range. Hooked up to the Low impedance taps, the Innersound Isis sang sweetly with superb timbre fidelity. And soundstage transparency was spectacular. The BAT seemed totally unfazed by this load and coaxed plenty of dynamics from a speaker that usually requires solid-state muscle amplification

Kudos to the BAT team for launching the all-tube VK80i. It is above all else a superb demonstration of triode power. If you’ve been mired in an audio rut, living with a cookie-cutter push-pull beam power or pentode amp, listen up. Sonic happiness in a glass bottle is within reach. You owe it to yourself to audition the VK80i. You’ll discover as I did that there is no turning back. 

Specs & Pricing

Output power: 55Wpc into 8- and 4-ohm loads
THD at full power: 3%
Frequency bandwidth: 8Hz–200kHz
Tube complement: 4x 6SN7, 4x 6C33C-B
Input sensitivity: 1.3 V
Input impedance: 100k ohms (1kHz)
Inputs: 1x XLR, 3x RCA
Volume control: Resistive ladder, 90-steps, 1dB per step
Dimensions: 17″ x 8″ x 16″
Weight: 45 lbs.
Price: $9995

BALANCED AUDIO TECHNOLOGY
1300 First State Blvd. Suite A
Wilmington DE 19804
(302) 999-8855
balanced.com

Associated Equipment
Speakers: Fyne Audio F1-8, Fleetwood Sound Company DeVille; Tannoy System 1000; Audiostatic ES-240; Innersound Isis 3.5
Phono front end: Bang & Olufsen Beogram 8000  turntable w/ MMC 20CL cartridge; Pentagon phono stage; Revox B795 turntable; TPAD 1000 phono stage; Sound Tradition MC-10 step-up transformer
Digital front end: Audirvana 3.5 software; Qobuz Streaming; audiolab 6000CDT transport; Audio Note 2.1 Signature, Denafrips Terminator; Soekris dac1421 DACs; Matrix Audio X-SPDIF 2 USB bridge; Uptone Audio ISO REGEN; Alldaq ADQ-USB 3.0 isolator
Cable & interconnects: Acrotec, Mogami & Kimber KCAG interconnects; Acrotec 6N, Analysis Plus Oval 12, & Take Five Audio Cryo treated Mogami 3103 speaker cable
Accessories: Sound Application CF-X & TT-7 power line conditioners